AD6402

–4–

REV. 0

AD6402

VCO

VCOGND

TXOUT

TXOUTB

MODOUT

FMMOD2

FMMOD1

1nF

1nF

SMV

1204-36

39nH

8pF

47pF

1.2k

Ω

4.7nF

1nF

SMV

1204-37

240

Ω

150pF

VCCI

330

Ω

100pF

330

Ω

180pF

330

Ω

TXMOD

TXIF+

TXIF–

Figure 1.

OVERVIEW

The AD6402 forms the basis of a highly integrated RF trans-

ceiver with the benefits of increased sensitivity and wide dy-

namic range that a dual-conversion architecture provides. The

IC contains a low dropout voltage regulator to isolate the IF and

demodulator VCOs from variation in the battery voltage, such

as power-supply transients caused by the PA. The AD6402 also

provides control circuitry that allows subcircuits to be turned off

and on as necessary to minimize power consumption.

Operation During Receive

The AD6402 contains the second mixer, integrated second-IF

bandpass filter, logarithmic-limiting amplifier, and PLL de-

modulator. A SAW IF bandpass filter is usually required at the

IF input in order to provide channel selectivity.

The placement of the SAW filter in the signal path between

the AD6402 and the RF section and the partitioning of the

receiver’s RF and IF receive circuits minimizes the leakage

around the SAW filter and maximizes the RF to IF isolation.

The output of the SAW filter enters the AD6402 via the second

downconversion mixer. This mixer is a high gain, doubly-

balanced Gilbert-cell type. The mixer downconverts the signal

to the second IF, which is 1.5

× or 2.5 × the reference frequency.

This multiple is determined by the state of the REFSEL pin. An

on-chip two section bandpass filter provides additional selectiv-

ity to provide attenuation of adjacent channels. The VCO con-

trol voltage output of the PLL demodulator tunes this filter to

the second IF.

The bandpass filter’s output enters a successive-detection loga-

rithmic-limiting IF amplifier. The RSSI detectors are distrib-

uted across the entire IF strip, including the mixer, and provide

80 dB RSSI range. The IF strip’s limiting gain also exceeds 80

dB. The RSSI signal is low-pass filtered and proceeds off-chip

to the baseband subsystem. The limited output of the logarith-

mic amplifier enters a PLL demodulator, which provides de-

modulation of the received signal. The PLL uses an integrated

VCO with no external components.

Operation During Transmit

The transmit signal path consists of a low-pass filter that can be

user configured for antialiasing of a baseband transmit signal.

An IF VCO, which should be tuned to a frequency equal to the

receive IF frequency plus the desired demodulator input fre-

quency, may be open-loop modulated by the transmit signal for

FM and FSK schemes. The receive IF mixer uses high side

mixing and therefore the IF VCO should be set to a frequency

equal to the sum of the IF frequency plus the frequency of the

PLL demodulator input as defined by the reference clock

divider ratios.

The transmit IF VCO uses an external tank circuit. This signal

is upconverted to the transmit frequency in the RF mixer sec-

tion of the radio. Using a transmit IF VCO prevents two prob-

lems: feedback from the PA at the RF frequency does not cause

distortion in the modulating circuit because the frequencies are

widely separated and the IF tank circuit can be optimized for

modulation linearity.

The output of the transmit VCO passes through buffer amplifier

and leaves the AD6402 via an optional LC filter between the RF

and IF ICs. The output of the LC filter may then be fed to a

transmit upconversion mixer for conversion to the final RF

frequency.

Onboard Voltage Regulation

The AD6402 contains a low dropout voltage regulator to spe-

cifically isolate the VCOs and synthesizer from the voltage

“kick” that occurs when a power amplifier switches on and the

battery voltage abruptly drops. The AD6402 uses an integral

vertical PNP pass transistor.

The regulator in the AD6402 IF IC supplies the voltage for the

VCOs on both the RF section and AD6402. The other sections

of the AD6402 should be powered from an independently regu-

lated source at 2.85 V. Since the VCOs are isolated from this

source, possible problems due to VCO supply pushing are con-

siderably reduced.

Frequency Control

The AD6402 requires an external synthesizer to provide the

control voltages for the tank circuit of the IF VCO. Normally

this will be the IF section of a dual synthesizer controlling both

IF and RF frequency generation.

It is recommended that the VCO on the RF section implement

the channel selection on transmit and receive; the VCO on the

AD6402 may therefore operate at a fixed frequency. This ac-

complishes two goals: first, the IF VCO being modulated can be

optimized for modulation linearity and the RF VCO can be

optimized for tuning range, and second, feedback from the PA

at will not couple into the modulating circuit to cause spurious

responses.

All key sections of the AD6402 may be powered up or down as

necessary to minimize power consumption and maximize

battery life.